Freezing out contaminant methane in the recovery of hydrogen from industrial gases



United States Patent O 3,315,475 FREEZING OUT CONTAMINANT METHANE IN THERECOVERY OF HYDROGEN FROM INDUS- TRIAL GASES Alexander Harmens, Purley,Surrey, England, assigner to Couch International Methane Limited,Nassau, Bahamas, a Bahamian company Filed .lune 15, 1964, Ser. No.375,131 Claims priority, application Great Britain, Sept. 26, 1963,37,870/ 63 9 Claims. (Cl. 62-12) This invention relates to the recoveryof hydrogen from industrial gases containing appreciable amounts ofhydrogen and methane, as for instance, coke-oven gas, coalgas, oil gas,derived for instance from a gas Oil, oil refnery cracking gas,carburetted water gas, and so on. The recovery of hydrogen fromcoke-oven gas is a particular attractive possibility on` an economicbasis, and provided methane is not lost; and the invention will 'be moreparticularly described hereinafter by reference to a coke-oven feed gas.

The present invention which aims at this recovery stems from theavailability in bulk of liquefied natural gas, which term is hereinintended to include also liquefied methane and is throughout hereinafterreferred to as L.N.G.

This product is becoming available in bulk, being transshipped by oceantanker from gas producing areas, as for instance in the Sahara. Theutilization of the considerable energy which is represented in the lowtemperature of large quantities of the liquefied fuel gas constitutes amajor objective in the present invention.

A further objective is the utilization of L.N.G. to recoversubstantially pure hydrogen and to replace it by a valuable fuel gas inquantities broadly comparable with the hydrogen recovered.

According to the present invention, in a process for the recovery ofhydrogen from an industrial gas also containing methane, the latter ispartially liquefied, the remaining gaseous fraction Washed in sub-cooledL.N.G. in an absorber, from which hydrogen contaminated only by minoramounts of methane is recovered, 'and the liquefied fraction combinedwith the liquid absorber bottom product is caused to serve asrefrigerant for partially liquefying incoming feed gas. Hydrogen leavingthe absorber is caused to lose further heat by expansion, for instance,in a suitable expansion machine, and thereby any contaminating methanecarried over is caused to solidify out which can be recovered forinstance in a cyclone. The purified hydrogen thus cooled may servethrough heat exchange -to sub-cool the stream of L.N.G., or part of saidstream passing to the absorber.

The refrigerant comprised by the mixture of the aforesaid liquefiedfraction and liquid absorber bottom product may be passed to anevaporator Where it is caused to boil by a steady withdrawal of vapor toa compressor, and by partial cycling off to the chilling section. Inthis evaporator a tube bundle carrying the feed gas, and if desired afurther tube bundle carrying the L.N.G may lbe disposed. The feed gasybefore entering the evaporator may advantageously be partially purifiedthrough heat exchange with returning hydrogen and/ or fuel gas, wherebyheavier constituents such as Water and/or carbon dioxide are recoveredas solid products. In the evaporator partial liquefaction of theincoming feed gas occurs and separation of the feed into two phases maytake place in a phase separator from which the gaseous fraction passeson to the bottom of an absorber.

Since absorption requires a wider temperature differential betweensub-cooled L.N.G. and incoming feed, the gaseous product may be allowedto gain some heat through heat exchange'with a side-stream of hydrogen,which in turn has received heat, for instance through cooling theincoming L.N.G on its way to the evaporator.

The absorber takes out remaining constituents in the gaseous fractionother than hydrogen, which passes overhead usually contaminated with asmall amount of methane. The liquid absorber bottom product is mixedwith the liquefied fraction of the feed and the combined product passedto the aforesaid evaporator. Herein a part of this combined product isevaporated whereby heat is abstracted from the incoming feed gas andfrom incoming L.N.G., and a lower' equilibrium temperature in theevaporator is established. The remaining cold liquid, suitablypressurized, may be cycled back to cool and purify the incoming feed gason its way to the evaporator. Fuel vapor and liquefied fuel cycled offfrom the evaporator after cooling incoming feed gas may thereafter becombined in one stream and any surplus cold left in this combined streammay be used as a pre-cooling medium for the feed gas.

The recovered hydrogen may likewise assist and supplement the liquid orgaseous fuel as :a coolant in purifying hea-t exchangers and inpre-cooling heat exchangers. Moreover, any methane recovered as solidproduct from the hydrogen from the absorber may be mixed with fresh feedand thus also contribute to the cooling of the latter, as willhereinafter be described.

The specific nature of the invention, as Well las other objects andadvantages thereof, will clearly appear from a description of apreferred embodiment, as shown in the accompanying drawing, in which:

The figure is a schematic fiow sheet fora plant for the recovery ofhydrogen from a coke-oven gas.

A stream of coke-oven gas having the approximate composition Mol percentwas cooled by a water cooler 1 to 65 F. and by a further pre-cooler 2,wherein it was refrigerated by surplus high level cold in returning fuelgas to 35 F. The gas stream passed on to a 3-st'age compressor 3,wherein it was compressed successively to 74, 184 and 460 p.s.i. withinter-cooling against water in a bath 4. The compressed gas finallyemerged at a temperature of 65 F. and then passed to the chillingsection.

In the first heat exchanger 5 of the chilling section the gas stream wascooled to F. against returning streams of purified hydrogen and of fuelgas. This chilling was followed by further chilling in one of a pair ofpurifying heat exchangers 6 and 6a. Here cooling against returningstreams of purified hydrogen and of fuel gas reduced the temperature toF. and carbon dioxide separated out as a solid. When necessary onaccount of carbon dioxide deposition, switching was effected to thesecond clean heat exchanger and the solid carbon dioxide deposit waspurged with air. Thereafter the gas at a temperature of -190 F. andcontaining only traces of carbon dioxide passed to the central port ofcyclone 7 or cyclone 7a, which cyclones, as suggested by the broken line20 Were coupled in a manner hereinafter set forth. The purified gasemerged at -200 F. One further heat exchange in heat exchanger 8 againstthe returning stream of purified hydrogen reduced the ternperature to210 F. and the gas then passed into the warm side of an evaporator 9.

' Vtemperature of 273 F. was reached,

In this evaporator the purified gas was chilled to 265 F. .and therebyVpartially liquified while passing through a tube bundle (not shown) inthe evaporator. The partially liquified gas stream emerged via line 1.0Qto a phase separator 11, wherein a liquid fraction containing about 2%hydrogen, 20% nitrogen and 78% methane separated out from a gas fractioncontaining about 88% hydrogen, 9% nitrogen'and 3% methane. The gasfraction from/this phase separator passed to a heat exchanger 12,wherein it was raised to a temperature of 250 rj. against a side streamof purified hydrogen. From this L.N.G.-in this particular instance,liquefied Vmethane at 259 F.-was pressurized to 460 p.s.i. in booster 13and thereafter passed Vvia line 14 into evaporator 9. On transversing atube bundle (not shown) in the Vevaporator the LNG. was sub-cooled to267 F. and

emerged via line 15. It then passed to heat exchanger '16' whereinit wascooled against an expanded cold stream of purified hydrogen to atemperaturerof 290 F. At this temperature it entered the top of absorbercolumn 17. In this column the'partial purified cokeover gas feed waseffectively scrubbed to remove nitro` gen. As will fbe seen the liquidabsorber bottom product present was yfrozen and can be removed onpassing to cyclone 7a.

As already indicated, cyclones 7 and 7a were coupled Y together andcould he switched to perform successively Vtwo different functions.frozen methane, cyclone 7 vaporized any solidied While cyclone 7acollected methane deposited therein, this being vaporized and taken 4upby the incoming coke-.over gas feedwhich was thus Y further cooled.Cyclone 7a in due course was switched Ainto ,thetincoming feed gas linein .place of Vcyclone 7 which thereupon assumed the former role ofcyclone 7a,

namely, recovery of methane deposited from the sub-V *cooledV hydrogenstream.`

Y heat exchanger the gas fraction passed into absorber |17 operating atabout 44() p.s.i.

The purifiedhydrogen emerging from the cyclone then Y Y Y served tosub-cool L.N.G.Y being fedto the absorber in heat exchanger 16. Byreason of this heat exchange, the

. Vterneprature of the stream rose to 276 F. It then combined with aside stream of re-cycled hydrogen which i Y yhad given up heat to thegaseous fraction of the feed in Vheatfexchanger 1.2, and the combinedstream assumed a temperatureof 269 F. For theY purposes ofrefrigeratingV both incoming coke-oven gas andf L.N.'G.Yin theevaporator, the mixture of the liquefied fraction inV phase Y Yseparator11Yand liquidY absorberl bottom product -thisV mixtui'ehaving roughlythe composition 3% hydrogen,

15% nitrogen and A82% methane-was passed via line operate at 9 p.s'.i.and at Va temperature of 273 F.

' This reduced pressure was maintained by drawing off Y Vvapor throughcompressor 28, which compressedl this vapor to 160 p.s.i. withoutinter-cooling. Thereby Vthe remaining liquid was caused to boil and anequilibrium Roughly one half ofthe tmaterialV entering the evaporatorremained inthe liquid phase, this liquid being a mixture of methane andnitrogen. With the aid of booster 24 this liquid was pressurzedV to 160p.s. i. and cycled fvia line 25 to purifywing heat exchangers 6 or 6awhere it served to purify` incoming feed by freezing carbon dioxide fromthe latter. The returning puried hydrogen stream in line 31 `21 andthrottle valve 22 into evaporator 9 designedV toV served as coolant inthe final heat exchanger 8 immediately before the evaporator 9 and alsoasan additional coolant in purifying heat exchangers 6 and 6a. A sidestream of this hydrogen passed via line 26 and booster 27 to heatexchanger 12 and then merged with the purified stream coming from heatexchanger 16. Y Surplus cold still remaining in the purifiedrh'ydrogenstreamY after coming from heat exchangers 6 and'cz was usedjforV furthercooling of Yincoming feed in heat exchanger 5.

The liquid fuel passing from the evaporator via line...

25 to heat exchangers 6 V0r6a emerged in line 25a from the latterV onlypartially evaporated at a temperature of 188 F. and this Z-phase streamwas combined in vessel 28 with the Ycompressed gasY vapor fromcoinpressor 23. Here any liquid phase evaporated and the combinedstreamassumed a temperature of 1389 F. it'was in part used as coolant in heatexchanger 5 and; in part in heat exchanger 2 and thereafter the dividedstreams were recombined having lost substantially all" YV surplus cold.

In a modification of the foregoing arrangements the refrigerating liquidin evaporator 9 was pressurized first to l60'p.s.i. then to 460 p.s.i.and thereafter fed into the incoming L.N.G., the rate of supply of thelatter being a VVcorrespondingly reduced. The'arnount fed back was so Vadjusted that the'remaining liquid used for refrigerating in theVchilling section was just sufficient to operate heat exchangers 5 and 6or 6a and pre-*cooling in heat ex-V changer' 2 was omitted. Y n Theforegoing modification can be put into effect in the drawing by openingvalve 30 and operating booster 33; in

practice, if this modication is employed, heat exchangerVV 2 can beomitted.

' Manifestly in the chilling sectionthe surplus cold Vfrom purifiedhydrogen and refrigerated fuel `gas can be `used in other ways fromthose set forth'above without departing from the present invention.

T he' installation in the treatment of 10 standard cubic 'Y feet per daycoke-oven gas used 750 long, tons per dayof L.N.G. and yielded 53x106standard cubicfeet per day of'hydrogen and 80 106 standard cubic feet.fuel gas. The cold in the L.N.G. contributed 43% Vof the energyYrequired for separation.

It will vbe apparent that the embodiments Yshown'arerVK only exemplaryand that various modifications .can be made in construction andarrangement within lthe Vscope l of myrinvention asdelined in theappended claims.

I claim:

. 1. Process for the recovery lof, hydrogen from industrial feed gasessuch as cokeoven gas, comprising the steps of (a) partially liquefyingthe feed gas, Y 1 (b) separating the partly liquefied feed gas intoafgase- Y ousfraction and a liquid fraction,V Y

(c)` sub-cooling a separate supply of liquefied natural gas, Y

'(d) washing saidV gaseous fractionin'the sub-cooledV Y liquefiednatural gas in Yan absorber and thereby rei i i covering hydrogen as amajor volatile constituent, contaminated only by minor amounts ofmethane,

(e) combining'saidV liquid fraction with the liquid abr-V.V l sorberbottom product from step (d), Y 1

V(f) passing the combined liquids from step `(e.) in heatexchangerelationship with the incoming feed gas in step (a) to partially liquefysame,

(g) expanding .said methane-contaminated methane as a separate solidproduct, and

(h) .using the expanded, methane-free hydrogen obtained for furthersub-coolingV the liquefied natural gas. Y

2. Process as claimed in claim 1, wherein the aforesaid Y' ycombinedliquids from step (e) used as refrigerantpforV mcommg feed gas servealso toYsub-cool liquefied natural.

gas being fed to the absorber.

3. Process as claimed in claim 1, wherein-methane hydrogen leaving theabsorber to freeze out the contaminating frozen out as a solid productfrom the -hydrogen stream from the absorber is combined with and coolsincoming feed.

4. Process as claimed in claim 1, wherein the methane frozen out as asolid product is recovered from the expanded hydrogen stream by means ofa cyclone.

5. Process as claimed in claim 1, wherein refrigeration of incoming feedgas is effected in an evaporator wherein the combined liquid fractionand liquid absorber bottom product is partially evaporated.

6. Process as claimed in claim 5, wherein a part of the combined liquidfraction and liquid absorber bottom product is cycled off to cool andthereby purify incoming feed gas by condensing out heavier components asa solid product.

7. Process as claimed in claim 5, wherein part of the combined liquidfraction and liquid absorber bottom product is brought to the samepressure as and cycled to the liqueed natural gas being fed to theevaporator.

8. Process as claimed in claim 1, wherein the methanefree hydrogenrecovered from the absorber, after subcooling the liquefied natural gas,is also used to cool i11- coming feed.

9. Process as claimed in claim 8, wherein a part of the hydrogen andafter expansion and heat exchange with liquefied natural gas andincoming feed gas, is cycled back to provide heat interchange with andlose heat to the gaseous fraction of the feed before it enters theabsorber.

References Cited by the Examiner UNITED STATES PATENTS 1,773,012 8/1930Schuftan.

1,830,610 11/1931 Linde 62-23 X 3,062,015 11/1962 Cost 62-17 3,197,9708/1965 Nelson et al 62-17 NORMAN YUDKOFF, Primary Examiner. V. W.PRETKA, Assistant Examiner.

1. PROCESS FOR THE RECOVERY OF HYDROGEN FROM INDUSTRIAL FEED GASES SUCHAS COKE-OVEN GAS, COMPRISING THE STEPS OF (A) PARTIALLY LIQUEFYING THEFEED GAS, (B) SEPARATING THE PARTLY LIQUEFIED FEED GAS INTO A GASEOUSFRACTION AND A LIQUID FRACTION, (C) SUB-COOLING A SEPARATE SUPPLY OFLIQUEFIED NATURAL GAS, (D) WASHING SAID GASEOUS FRACTION IN THESUB-COOLED LIQUEFIED NATURAL GAS IN AN ABSORBER AND THEREBY RECOVERINGHYDROGEN AS A MAJOR VOLATILE CONSTITUENT, CONTAMINATED ONLY BY MINORAMOUNTS OF METHANE, (E) COMBINING SAID LIQUID FRACTION WITH THE LIQUIDABSORBER BOTTOM PRODUCT FROM STEP (D), (F) PASSING THE COMBINED LIQUIDSFROM STEP (E) IN HEATEXCHANGE RELATIONSHIP WITH THE INCOMING FEED GAS INSTEP (A) TO PARTIALLY LIQUEFY SAME, (G) EXPANDING SAIDMETHANE-CONTAMINATED HYDROGEN LEAVING THE ABSORBER TO FREEZE OUT THECONTAMINATING METHANE AS A SEPARATE SOLID PRODUCT, AND (H) USING THEEXPANDED, METHANE-FREE HYDROGEN OBTAINED FOR FURTHER SUB-COOLING THELIQUEFIED NATURAL GAS.